The present invention relates to a piezoelectric device formed by a composite substrate which is bonded by wafer bonding or anodic bonding, and also to a manufacturing method of the piezoelectric device.
With regard to manufacturing a piezoelectric device used, for example, in a quartz oscillator, a method of bonding a piezoelectric substrate to a semiconductor substrate by wafer bonding or anodic bonding without using an intermediate adhesive layer has been recently studied. Wafer bonding is defined as a technique of joining two mirror finished substrates and applying heat thereto, thereby bonding the two substrates at an atomic level. Anodic bonding is defined as a technique of joining two mirror finished substrates and applying heat while applying a voltage to the interface between the two substrates, thereby bonding the two substrates at the atomic level.
For example, page 1045 of IEEE Ultrasonic Symposium Proceeding 1994 teaches wafer bonding of semiconductor substrates to piezoelectric devices. For example, silicon and quartz are bonded at the atomic level using a sandwiched layer of silicon dioxide therebetween. The silicon dioxide is not an intermediate adhesive layer but a product of atoms making up a silicon substrate or a quartz substrate. Another example is found in Applied Physics Letters vol. 66, page 1484 (1995). In that reference, two substrates of the same material, such as lithium niobate, are bonded by wafer bonding at the atomic level.
Wafer bonding requires, for example, the following steps:
(a) mirror finishing the bonding faces of substrates, PA1 (b) washing the substrates, PA1 (c) hydrophilically treating the substrates, if necessary, PA1 (d) joining the two substrates, and PA1 (e) applying heat and bonding the two substrates. PA1 (a) rough surfaces, swell, warpage, and/or strains of the substrate; PA1 (b) particles on the bonding interface; PA1 (c) gases enclosed in the bonding interface; PA1 (d) water molecules and atoms attached thereto enclosed at the bonding interface, when water molecules and atoms attached thereto are used for initial bonding.
However, if two substrates undergoing wafer bonding or anodic bonding have different thermal expansion rates, e.g. quartz and silicon in the manufacturing of a quartz oscillator, the substrates may, due to the different thermal expansion rates, break or delaminate in step (e) when heat is applied. Japanese laid-open patent application H5-327383 discloses that a close relationship exists between the thickness of a substrate and damage or delamination thereof, when two substrates, having different thermal expansion rates, are bonded by wafer bonding or anodic bonding.
Further, damage to substrates depends on the total area bonded. A maximum heat treatment temperature (MHTT) at which no delamination occurs is defined as the temperature at which a composite substrate undergoing bonding withstands damage due to thermal stress. FIG. 11 shows that MHTT depends on the total area bonded. The horizontal axis represents a bonded area on the substrate, and the vertical axis represents normalized MHTT of the composite substrate, where a temperature value is normalized by the MHTT of 1600 mm.sup.2 of bonded area.
FIG. 11 shows a curve plotted from experimental values depicting the relationship between a bonded area and MHTT, where a quartz substrate was bonded to a silicon substrate by wafer bonding through the process of hydrophilically treating, joining and heat treating. In the experiment, the quartz substrate had a thickness of 100 .mu.m and the silicon substrate had a thickness of 600 .mu.m. Both substrates were square. The bonding process proceeded as follows: first, the two substrates were hydrophillically treated with a mixed solution of ammonia, hydrogen peroxide, and deionized water. Second, the two substrates were joined and the temperature raised at the rate of 100.degree. C./hour. Finally, the two substrates were held for two hours at the maxi mum temperature.
Since MHTT varies depending on the thickness of the substrates and the overall bonded area, it is difficult to bond two substrates of different materials by wafer bonding. Accordingly, wafer bonding has not been easy to employ in manufacturing piezoelectric devices.
MHTT is impacted by uneven distribution of thermal stresses applied to the bonded faces during the heating process. The following factors may be involved.
Bonding strength is not distributed evenly across the bonded face during the heating process. In wafer or anodic bonding, the bonding speed at the interface is not constant, that is, one area may be more strongly bonded than another. Accordingly, bonding stress is distributed unevenly in the substrate.
Also some areas may not bond altogether. That is, one area may be bonded within the substrate while another is not. Accordingly bonding stress is likewise distributed unevenly in the substrate. The following are possible reasons why some areas remain unbonded:
An unbonded area is referred to as a void hereafter.
Sometimes piezoelectric substrates are apt to be electrically charged, which leads to particles being easily attached thereto. Such a substrate is vulnerable to damage and delamination for reason (b) set forth above. To be more specific, micron-sized particles can attach to the bonding interface, whereby voids are produced. The voids lead to extensive substrate delamination, whereby the substrate cannot be used. It is difficult to remove all of the micron-sized particles from the insulated and charged substrate, and if additional removal means is added, the washing process can become complex. When insulated substrates having anisotropic thermal expansion rates are used, the orientations of such substrates should be precisely matched. If not, damage and delamination due to stress will result.
The problems associated with conventional heat treatments in the bonding process are described above. These same problems are found in the manufacturing of piezoelectric devices. That is, when a piezoelectric device undergoes a heat-applying-process such as soldering reflow for the piezoelectric device to contact an outer electrode, the same phenomenon is observed, and piezoelectric composite substrates are delaminated. Even if voids in the bonding interface do not damage the substrates, the voids still apply stresses to piezoelectric devices formed on the piezoelectric composite substrates, thereby changing the characteristics thereof. Moreover, the voids may lead to a decrease in the reliability of the piezoelectric devices.